chainset/BB/front mech issues?

The Big Cheese
The Big Cheese Posts: 8,651
edited May 2011 in MTB workshop & tech
Swapped an FSA 48/36/26 chainset on my BadBoy to a Shimano Alivio Square Taper (which is 42/32/22).

Square taper BB, I thought I could swap them straight over. However.

Although the BB looks OK and the cranks don't appear 'off' the shiting didn't work after I reconnected the chain.

Questions:

Does the crankset 'specifically' require a Shimano UN54 BB (110mm) and what would happen if I didn't use that model - just the chainline being out?

Also, it appears as now I have smaller rings, the top swing mech is too far up (the seattube and looks too high compared to the big ring), and I cannot move it further down the seatube as it's almost sitting on top of the BB shall as it is.

Now I know that a frame is not designed around a specific chainset, but it seems the mech might be... top swing for the larger rings, but perhaps a 'conventional' for the smaller set up.

I've left it for tonight. However I shall need to order the new BB soon as I want to cycle the weekend.

Any ideas?

Comments

  • nicklouse
    nicklouse Posts: 50,675
    Square taper cranks need the correct length of axle to give the correct chain line.

    from 110mm to about 128mm

    also if you have changed from 44 to 42 teeth you you will need to move the mech down and re do the cable. if it is frayed it will most likely need replacing.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • The Big Cheese
    The Big Cheese Posts: 8,651
    nicklouse wrote:
    Square taper cranks need the correct length of axle to give the correct chain line.

    from 110mm to about 128mm

    also if you have changed from 44 to 42 teeth you you will need to move the mech down and re do the cable. if it is frayed it will most likely need replacing.

    110mm UN54 ordered.

    FDront mech wont slide down the seatube anymore as it will foul the BB shell/welds.

    I have room for a conventional, would I be better off with one of them?

    Oh and I went from 48 to 42 ;)
  • nicklouse
    nicklouse Posts: 50,675
    sounds like a new mech will be needed.

    check the axle length required for the new crankset.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • The Big Cheese
    The Big Cheese Posts: 8,651
    nicklouse wrote:
    sounds like a new mech will be needed.

    check the axle length required for the new crankset.

    checked shimanos webby and it said 110mm axle length UN54 BB (68mm shell)

    Bummer about the new mech.. but hey ho :(

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